This section is intended to provide a background or context. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the appended claims and is not admitted to be prior art by inclusion in this section.
In wireless communication systems, multiuser diversity capitalizes on independent fading channels across different users in order to enhance the throughput in the downlink/uplink of a cellular system. Serving the user with the best instantaneous channel quality has been shown to be optimal in terms of ergodic sum-rate for both the uplink and for the downlink. However, it requires all users to feed back their instantaneous channel state information (CSI) to the transmitter. It has been shown that, given this optimal scheduling, the ergodic sum-rate capacity of the downlink Rayleigh fading channel scales as log log K with the number of users K.
The optimal scheduling discussed above has at least two problems. First, there is a large amount of required feedback. Second, the feedback delay may cause the CSI fed back to the base station to be outdated. In order to reduce the feedback load, various schemes have been proposed. One approach prescribes feedback of a quantized version of the CSI. A 1-bit feedback scheme was proposed (without considering feedback delay). According to this scheme, for each fading block, users with channel power exceeding a given predetermined threshold feed back the bit “1”, otherwise they indicate “0” to the base station. The base station randomly chooses one among the users with feedback bit “1” for data transmission with power P. When there is no user signaling a channel gain larger than the threshold, the transmitter keeps silent for one block period. This 1-bit feedback scheme suffers from a negligible loss of multiuser diversity gain as compared to the full CSI feedback scheme. In particular, the optimal scaling law of log log K is preserved.
In a realistic situation, it is impossible for the scheduler at the base station to access the instantaneous (and possibly quantized) CSI of each user. In fact, channel feedback information may become outdated if the fading channel is changing rapidly. This leads to a degradation of the system sum-rate. For instance, in case of full CSI feedback, the “best” user may no longer be the “best” after a feedback delay.